Of course, slavery is not part of an optimal system. But the point is that accepting it as part of an existing culture or economy does not discredit one's arguments on other issues--such as some of those that were also in play in the period that you have chosen for your strident insults.
Very cute, you get uppity over my observance of your hyperventilation and (again) try to put words in my mouth. Maybe posting on a political site is too strenuous for you?
I noticed that too. People are fond of asserting modern sensibilities in judging people of times past. In this case he actually acknowledges that the people of that time had a very different understanding of the moral positions involved than what we do today. Even Abraham Lincoln said some things about Blacks that would make people of today cringe.
Most of the time the issue of slavery is an attempt to assert that their position is correct because "Shut up! The Losers owned slaves!"
It is a technique intended to shut down a dispute rather than to address the points brought forth by it. It is basically an ad hominem fallacy.